1. Field of the Invention
The present invention relates to a separator membrane for use in secondary alkali batteries. More particularly, the present invention relates to a separator membrane for use in secondary alkali batteries that is dimensionally stable and which retains a constant electrical resistance in a hot and concentrated aqueous alkali solution.
2. Description of the Prior Art
Secondary alkali batteries (e.g. Ni-Zn and Ni-Cd) use a concentrated KOH or NaOH aqueous solution as the electrolyte and require a separator that has to meet essentially the same requirements as are fulfilled by the separator used in silver oxide primary batteries. In secondary alkali batteries, the temperature of the electrolyte may exceed 100.degree. C. if they are repeatedly charged and discharged at high rate within a short period. If, under this hot environment, the electrical resistance of the separator is increased or its dimensions are changed, the batteries performance is reduced.
Therefore, the present inventors have made various efforts to produce a separator membrane which experiences minimum swelling in a hot and concentrated alkaline aqueous solution.
Ion exchange membranes are important as a separator indispensable to electrolysis and electrodialysis. Most of the conventional ion exchange membranes are composed of a styrene-divinyl benzene or a styrene-butadiene copolymer having a sulfone group or a quaternary vinyl pyridinum salt introduced therein, but because of their inadequate mechanical strength and chemical resistance, they find only limited utility.
With recent advance in technology for manufacture and application of ion exchange membranes, various ion exchange membranes have been developed and their use is increasingly expanded. An ion exchange membrane made of a fluorine-containing polymer using a sulfone group or carboxyl group as an exchange has been found to be useful as a separator for brine electrolysis in the manufacture of caustic soda or as a high-molecular electrolyte for water electrolysis, and it has also been found that an ion exchange membrane prepared by grafting acrylic acid or methacrylic acid onto a polyethylene film has good properties for use as a separator in an alkali batteries using aqueous KOH or NaOH as an electrolyte. But one great problem of these ion exchange membranes is that their electrical resistance is increased with increasing temperature in a relatively high concentration of aqueous KOH or NaOH. To reduce the theoretical decomposing voltage and also to lower the overpotential and ohmic loss due to solution resistance, brine electrolysis or water electrolysis is desirably performed at elevated temperatures. The temperature of secondary batteries is rapidly increased during high rate discharge and charge cycles, so they are required to have high heat resistance. So the separator for use in these applications is desirably such that its electrical resistance is not increased at elevated temperatures.